The invention relates to a coolable arched roof for a high-temperature melting furnace.
Arched roofs for high-temperature melting furnaces, e.g. glass-melting furnaces or electric furnaces, generally comprise a layer, usually a plurality of layers, of heat-resistant (refractory) bricks. These consist, for example, of silicon carbide fireclay, high alumina and/or chromium corundum. Particularly in the case of an unsupported arched roof, the refractory bricks have to be dimensionally stable even under a prolonged thermal load, to ensure that the roof holds. An additional load on the bricks is produced in installations which do not operate permanently, for example reduction melting furnaces in garbage incineration plants, as a result of heat-related contraction and expansion of the bricks. To prevent the service life of the refractory bricks from being shortened, therefore, it is essential to observe the maximum mean temperature taken across the entire layer, which is stipulated by the manufacturer.
The arched roof presents an additional problem in furnaces in which the furnace interior has to be both outwardly and inwardly sealed. By way of example, in reduction melting furnaces, no oxygen must be allowed to penetrate into the furnace from the outside in order not to impair the reducing atmosphere in the furnace interior and in order to prevent metals which have already been reduced from being oxidized again. Furthermore, reducing gas should not penetrate to the outside, since it condenses on cooler, in particular, metallic components and accelerates the corrosion of these components. The seal provided by the refractory layer is further, reduced by wear to the refractory bricks caused by high levels of thermal loads and fluctuations in heat which can lead to deformation of the brickwork.
DE 27 58 755 has disclosed an arched roof which is water-cooled in order to increase the service life of the refractory bricks. The arched roof comprises an arched ring on which a framework of pipes for a coolant is supported. The refractory bricks are laid loosely on to the pipes. The cooling protects the refractory bricks from an excessively high thermal load. However, the arched roof is not sealed. Moreover, the pipes are directly exposed to the furnace atmosphere.
Therefore, the invention is based on the object of providing an arched roof which is durable and seals the furnace interior against the penetration or leakage of gas.
The coolable arched roof according to the invention comprises, in addition to at least one layer of refractory bricks, on the side thereof which is remote from the furnace interior, at least one sealing layer, an insulation layer with a thermally insulating action and a cooling layer which is designed to carry a cooling fluid.
The sealing layer is used to seal the interior of the furnace against leakage or penetration of gas. It preferably comprises a metal foil. A steel foil, which is preferably reinforced by a glass fiber fabric, is particularly suitable. A sealing layer of this type does not burn or melt at the temperatures of 100 to 450xc2x0 C. which prevail on that side of the bricks which is remote from the furnace interior and also withstands the excess pressure in the furnace interior. The sealing layer may also be arranged within the refractory layer, for example in the case of a layer structure comprising refractory bricks and light refractory bricks or light refractory plates, may be arranged between the sub-layers formed therefrom, which can then also act as an insulating layer.
To prevent excessive heat losses from the furnace, the sealing layer is separated from the cooling layer by the insulation layer. The insulation layer is used to maintain a predetermined temperature difference between the exterior and interior and between the refractory bricks and the environment. Keeping the sealing layer within a temperature range which does not fall below a predetermined minimum temperature additionally prevents the condensation of aggressive gases on the sealing layer. A predetermined quantity of heat is dissipated via the arched roof by the cooling fluid in the cooling layer.
Therefore, in the arched roof according to the invention, the layers interact in a very advantageous way in order to ensure the durability and seal of the arched roof over the maximum service life of the furnace. In the invention, in particular the mean temperature of the refractory bricks is controlled by targeted dissipation of heat and by building up a temperature gradient from the inside outward.
It is preferable for the thickness and material of the layers and/or the dissipation of heat effected by the cooling fluid to be selected in such a manner that the mean temperature in the refractory bricks does not exceed a predetermined temperature. This temperature is preferably between 1300 and 1600xc2x0 C., and in particular is approx. 1450xc2x0 C. The sealing layer is also held within a predetermined temperature range, which is above the dew point of the aggressive gases which are present in the furnace interior. The minimum temperature is preferably 150-250xc2x0 C., particularly preferably 200xc2x0 C. The insulation layer can preferably maintain a temperature difference of 100 to 300xc2x0 C., preferably 200xc2x0 C. At its surface, it is still preferable for a temperature of 100 to 200xc2x0 C. to prevail. The cooling layer is preferably in contact with this surface over the entire area of the surface. For this purpose, it is preferable for the cooling layer to comprise a covering layer and pipes which are connected thereto directly or indirectly via contact elements. The cooling layer dissipates a predetermined quantity of heat.
The invention is particularly suitable for furnaces with an unsupported arched roof, since in this case it is particularly important to maintain a predetermined mean temperature in the refractory bricks, for stability reasons. Furthermore, the invention is suitable for furnaces in which the gas atmosphere in the furnace interior has to be particularly well controlled, for example for reduction melting furnaces, in particular for those used to treat slag from garbage incineration.